Natural gas liquefaction



March 7, 1950 H. c. COOPER NATURAL GAS LIQUEF'ACTION Filed Aug. 18, 1945INVENTOR. Ham/44 C. COOPER WQQ QQMQRTE Patented Mar. 7, 1950 UNITEDSTATES PATENT OFFICE 2,500,118 4 Y NATURAL GAS LIQUEFACTION Howell C.Cooper, Sewickley, Pa. Application August 18, 1945, Serial No. 611,279

2 Claims. (Cl. 62-1755) This invention relates to the liquefaction ofnatural gases for storage or transportation purposes, and moreparticularly to liquefaction of such gases containing nitrogen insubstantial quantities, and constituting in fact a mixture of suchnitrogen and the methane which is the principal combustible constituent.

The principal object of the invention is to rid the liquefactionapparatus of the nitrogen necessarily received by it, with a minimumloss .of methane; it having been the practice in the art heretofore tosimply .vent the nitrogen under conditions entailing the loss ofrelatively greater amounts of methane.

Briefly, the invention contemplates in this respect liquefaction of theentire methane and nitrogen contents of the natural gas at a lowerpressure than would be necessary for liquefaction of the nitrogen only,so that a refrigerant such as methane is employable, utilizing itslatent heat in the usual manner.

Further objects of the invention are to conserve power in the process,as will appear.

Still further objects and advantages will be apparent from the followingdescription taken in connection with the accompanying drawingconventionally and diagrammatically illustrating a circuit embodying theinvention.

With reference now to the drawing, I is the incoming natural gas supplyline, which leads to the product condenser 2 with pressure determined bythe compressor 3 or equivalent pressure regulator.

The product condenser 2 is served with a refrigerant, which may bemethane, liquefied in the methane condenser 4.

The methane circuit includes, commencing with the methane condenser 4, amethane heat exchanger 5, expansion valve 8, flash tank 1, expansionvalve 8, product condenser 2, first storage compressort, intercooler l0,second stage compressor H with its intercooler l2 and third stagecompressor l3 with its intercooler l4, methane precooler l5 and back tothe methane condenser 4, it being understood that fiow in this methanecircuit is in the direction of the order of the parts recited, by way ofthe suitable connections indicated in the drawing.

Also, in the methane circuit is a bypass l6 by which gas from the flashtank I is passed through the heat exchanger 4 and returned to' theprincipal methane circuit ahead of the second stage compressor ll.

from which it emerges at 126, partial expansion in the flash tank 'Ifrom which the liquid emerges at 220, boiling within the productcondenser 2 at a temperature of 252", taking up heat therein from whichit emerges and enters the first stage compressor, as a gas, at 0,leaving the last cooler M at 100 and re-entering'its condenser 4 at a 8wherein it is reconverted back from a gas to a liquid. The temperaturein the bypasslii is --140.

Through this cycle the methane pressure will vary from 600 pounds, as itleaves its condenser 4 to 17 pounds as it leaves product condenser 2, aswill be appreciated.

The temperatures recited are in degrees Fahrenheit and the pressures inpounds per square inch absolute, but it is to be understood that theThis methane refrigerant circuit is generally a known one and itsoperation, briefly, includes liquefaction of the methane in itscondenser 4 figures are recited above and hereinafter only by way ofexemplification.

Liquefaction of the methane in its condenser 4 is accomplished by theemployment of ethylene as a refrigerant, which ethylene is liquefied inthe ethylene condenser 20. The ethylene circuit is generally similar tothe methane circuit, the ethylene. passing from its condenser 20 throughexpansion valve 2 I, flash tank 22, expansion valve 23, methanecondenser 4, first stage compressor 24 with intercooler 25 and secondstage compressor 26 with its aftercooler 21 and back to the ethylenecondenser; a bypass 28 being provided from the flash tank 22, as before.

The ethylene feeds its condenser at a -8, enters the methane condenserat a 145, leaves it at a 20 and re-enters its condenser 29 at a+ 100,its pressure running from 355 pounds in its condenser 20, to 17 poundsleaving the methane condenser.

The ethylene in turn is liquefied in its condenser 20 by the employmentof ammonia liquefled in the ammonia condenser 30 and employed in asubstantially similar circuit including expansion valve 3|, fiash tank32, expansion valve 33, ethylene condenser 20, first stage compressor34, intercooler 35, second stage compressor 36, aftercooler 31. However,this ammonia circuit preferably includes a bypass around the ethylenecondenser 20 and through the methane precooler l5, controlled by theexpansion valve 38. It also includes the bypass 39 leading from theflash tank 32.

The ammonia leaves its condenser 30 at 100, enters the ethylenecondenser 20 at 24, leaves the ethylene condenser at reenters theammonia condenser 30 at varying in pressure from 250 pounds in itscondenser to 17 pounds leaving the ethylene condenser.

The ammonia condenser is served by cooling water entering by line 40 andleaving by line 4| and which may be served by a coolingtower notillustrated;

The system described will be recognized as what is known as of thecascade type whereinjs employed another in a lower temperature range orbeing served by another in a higher temperature range, or both. I

The temperature range available from the methane, for condensation ofthe natural gas mixture in the product condenser 2 is such that only arelatively low pressure of the natural gas such as175 pounds isnecessary entering the condenser.

The methane temperature being fixed, the natural gas mixture pressuremay be adjusted dependent upon its nitrogen content so that only liquid,including liquefied nitrogen as well as liquefied methane will emergefrom the bottom of the product condenser as through the line 50, wherethe temperature may be 240 and pressure 175 pounds.

The nitrogen having been liquefied along with the methane, it isseparated from the latter in a separator 5| which enters by way of anexpansion valve 52.

The nitrogen separator 5| is in the form of a tower having the bubbleplates conventionally indicated and its operation depends upon anitrogen liquefaction circuit whereby a liquid nitrogen spray isprovided at the upper part of the tower by which gaseous methane iscondensed and thereby rejected from a gaseous nitrogen outlet at thetower top.

The nitrogen liquefaction circuit includes the nitrogen condenser 60wherein the nitrogen is liquefied, expansion valve 6|, flash tank 62,expansion valve 63, separator 5|, nitrogen precooler 64, interstagecooler 65, compressor 66, intercooler 61, interstage cooler 65, secondstage compressor 68, aftercooler 69, precoolers l and 64 and back to thenitrogen condenser 60. A bypass connection 1| leads from the flash tank62, by way of the precooler l0, and back into the circuit ahead of theinterstage cooler 65.

The nitrogen condenser 60 is served by methane from the methanecondenser 4 by a circuit which bypasses the product condenser 2 andincludes the line 80, expansion valve 8|, nitrogen condenser 60, line 82and product condenser 2.

In the nitrogen liquefaction circuit the nitrogen leaves its condenser60 in liquid form at a --240, leaves the flash tank 62 at a -282, leavesthe separator 4| at a 310, the precooler 64 at a 70, the interstagecooler 65 at +70, the first stage compressor at +320", the interstagecooler 55 at 50, the second stage compressor 68 at +155", theaftercooler 69 at +100, the precooler 70 at -58 and reenters thecondenser 60 at 239. The pressure within the nitrogen separator is 25pounds, which is maintained substantially up to the first stagecompressor 66, leaving the aftercooler 69 at 400 pounds.

Within the separator 5| the temperature of the nitrogen is at 310 whichis below methane liquefying temperature so that any methane whichemerges from the expansion valve 52 as a gas will be condensed and fallto the bottom of the separator where it will be at 260.

Nitrogen gasified in the separator 5| enters the described nitrogencircuit and its refrigerant capacity is largely conserved in thedescribed heat exchangers 64, 65 and I0. The nitrogen circuit a sourceof refrigerants, each servin is vented immediately ahead of its firstcompressor 66 under control of a valve 90 by which the nitrogen beyondthat necessary for operation of the circuit, is disposed 01'. Once thesystem is in operation, the amount of nitrogen thus vented will besubstantially the amount which enters the system by way of the supplyline it being understood that a slight amount of nitrogen will remain insolution in the liquid methane leaving the nitrogen separator 5|.

From the separator-the liquid methane is withdrawn subject to valve 9|to a storage tank 92 from which in turn it may be withdrawn asdesired-through line 93, the storage tank being provided with the usualvent or relief 94 to take care of evaporation losses.

In summary, attention is again called to the fact that the temperaturesand pressures herein above recited are by way of example only and mayvary somewhat according to various conditions, including. the capacityof the system. In the example shown, the capacity is in the order offour million cubic feet of gas per day deliverable to storage, theentering gas comprising a mixture of about 91% methane and 9% nitrogen,by volume, at a temperature of It may also again be observed that all ofthe incoming natural gas mixture is liquefied, under conditions verysubstantially less demanding than would be otherwise necessary forliquefaction of the nitrogen content alone; a common refrigerant sourceis employed for liquefaction of the mixture, and of the nitrogennecessary for separation; and purge of the nitrogen is had afterrecovery therefrom of most of its value as a refrigerant.

I claim:

1. In the production of liquid methane from a natural gas mixturecontaining a substantial amount of nitrogen: liquefying said mixture byheat exchange with a boiling refrigerant, separating nitrogen as a gasfrom the liquefied solution by reduction of pressure thereon in thepresence of liquid nitrogen, employing previously separated andreliquefied nitrogen for the purpose, and venting the resultant gaseousnitrogen in excess of that required to be reliquefied for continuationof the separation process.

2. In the production of liquid methane from a natural gas mixturecontaining a substantial amount of nitrogen: liquefying said mixture byheat exchange with a boiling refrigerant, separating nitrogen as a gasfrom the liquefied solution by reduction of pressure thereon in thepresence of liquid nitrogen, employing previously separated andreliquefied nitrogen for the purpose, and employing some of saidrefrigerant for said nitrogen reliquefaction.

HOWELL C. COOPER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

1. IN THE PRODUCTION OF LIQUID METHANE FROM A NATURAL GAS MIXTURECONTAINING A SUBSTANTIAL AMOUNT OF NITROGEN: LIQUEFYING SAID MIXTURE BYHEAT EXCHANGE WITH A BOILING REFRIGERANT, SEPARATING NITROGEN AS A GASFROM THE LIQUEFIED SOLUTION BY REDUCTION OF PRESSURE THEREON IN THEPRESENCE OF LIQUID NITROGEN, EMPLOYING PREVIOUSLY SEPARATED ANDRELIQUEFIED NITROGEN FOR THE PURPOSE, AND VENTING THE RESULTANT GASEOUSNITROGEN IN EX-